The overall objectives of this competing continuation application are based on the hypothesis that the blood-borne monocyte is the principal source of foam cells in the early atherosclerotic lesion, that hypercholesterolemia is associated with altered moncoyte production and circulation kinetics, and that hypercholesterolemic conditions induce the formation of monocyte subpopulations have altered structural and functional characteristics that are related to cellular mechanisms in athezo genesis. To test these hypotheses, this application proposes to determine whether in vivo monocyte production or circulation kinetics are altered during atherogenesis by examining the fate in the circulation or in the artery wall of monocytes labelled with H3-thymidine in vivo. Additionally, monocytes labelled in vivo with H3-thymidine or in vitro with FITC will be isolated and reinjected into other normal and hypercholesterolemic animals to document intimal penetration of monocytes, their transition into foam cells, residence time in the intima, and the possible migration of lipid-laden foam cells out of the lesion using fluorescent microscopy or autoradiographic and scintillation counting techniques. New data from two atherosclerotic models suggest two different mechanisms. Hyperchholesterolemia in the rabbit is associated with marked leukocytosis but not alteration in differential counts of blood leukocytes. In contract, the swine model shows no generalized leukocytosis, but rather, a marked monocytosis and neutrophilopenia. The monocytosis involves the formation of two structurally distinct monocyte subpopulations in addition to the normal monocyte. These three subpopulations of monocytes seen in hypercholesterolemic swine but not normal swine will be separated and isolated using counterflow centriguation (elutriation) techniques, and tested for cholesterol esterifcation ability when stimulated by modified LDL. Additionally, the ability of these subpopulations, and compared to normal monocytes and neutrophils, oxidize LDL and render it cytotoxic to arterial cells will be exameined together with indices of macrophage structure and function, inluding enzyme activities, glass adherence, and non-specific and immune-mediated phagocytosis. These aspects of monocyte function related to atherogenesis mechanisms will be studied in vitor anc correlated with in vivo lesion formation, monocyte recruitment into lesions, and monocyte circulation kinetics.